The stepping motor, which is founded on old, well-known, basic ideas, has found extensive use in recent years in connection with the further technical development of both construction of the motor itself and its current supply. The basic principles of the stepping motor and its drive, as well as at least a certain amount of further development are well known to one skilled in the art, and are therefore not described here. However, for one not skilled in the art, reference is made to the book "STEPPING MOTORS: a guide to modern theory and practice" by P. P. Acarnley, published by Peter Peregrinius Ltd for the Institution of Electrical Engineers as No. 19 in the IEE Control Engineering series.
For one skilled in the art, micro-stepping is a well-known method of giving the stepping motor greater resolution than full stepping. Micro-stepping is so well known in connection with stepping motors that it ought not to be necessary to describe it here. However, further to the book mentioned above the following three references may be helpful to one unskilled in the art: "Mini-stepping motor drives" by E. K. Pritchard, Proceedings of the fifth annual symposium on incremental motion control systems and devices, University of Illinois; "TECHNOLOGY of MICROSTEPPING, OEM DESIGN," October 1983, pp 69-71; and to "Microstepping: Small Steps Turn Into Big Improvements" by E. Slingland, POWER CONVERSION INTERNATIONAL, October 1983, pp 20-25.
Many drive circuits are known for supplying current to stepping motors from voltage sources and regulating the phase currents through the stator windings. It is also well-known in these cases that current decline can be different for different drive circuits and that different operating conditions can give differently rapid current decline in a single drive circuit. Of special interest in connection with the present invention are drive circuits including controllable switches, which are intermittently controlled to be in a conductive or blocking state, whereby the current through a stator winding can be regulated towards a desired value. The basic principles for such drive circuits and such regulation are well-known to one skilled in the art and are therefore not described here. The first and last of the above-mentioned references are recommended to one unskilled in the art, and apparatus and a method illustrated on page 23 in the POWER CONVERSION INTERNATIONAL reference, may be of special interest for comparison with a preferred embodiment of the present invention.